In electrostatic web printing or drum printing on sheets, such as in facsimile machines, an electrostatic latent image on the web or sheets is developed into a visible image, usually by applying a liquid suspension of toner particles to the sheets.
In U.S. Pat. No. 4,482,242 to Moraw et al., a latent charge image on a drum is developed by moving the drum through a pool of developer liquid in a developer holder. The excess liquid is removed by spillage off the edges of the holder, and by a fast moving roller contacting the drum.
In U.S. Pat. No. 4,157,219, Ohta et al. describe a multicolor electrostatic imaging apparatus wherein different color liquid developers are supplied to a medium bearing an electrostatic latent image. The apparatus comprises leading and trailing squeeze rollers in horizontally spaced relation, a receptacle having a plurality of valved pipes connected to respective liquid tanks for selectively applying a plurality of liquid developers and rinse liquid to the medium, and a corona charging device disposed at the rear side of the trailing roller for neutralizing or dissipating the residual charge remaining on the medium after development. These components are mounted to a reciprocating carriage so that the latent image is successively subject to developing, squeezing and charging during movement in a forward direction and to rinsing during movement in the reverse direction. Such movement is repeated some desired number of times to make a multi-colored print.
Many problems exist with conventional electrostatic printers. The supplies of developer must be frequently replaced since large amounts are applied during each printing of a sheet. The toning applicator must be prewet before it is used since it often dries out between use. Further, toning applicators, such as that in the patent to Moraw et al. above, must be periodically cleaned to remove toner particles caked or dryed onto the shoe. Since conventional toning applicators extend the width of the web or drum, they must be hand cleaned with large amounts of cleaning solvent, sometimes as frequently as once a day. This results in a considerably messy task and also results in a long time during which the printer is not in use. Color printers with multiple supplies of different color developers considerably complicate this task and may have the added problem of cross-contamination of developers.
It is an object of the present invention to provide a color printing method which is self-cleaning, is immediately ready ror printing without prewetting, and which provides for recycling of developer and solvent material, with a minimum of cross-contamination.